Filtering of analog signals often involves modulation or demodulation to move signals of interest from one frequency to another. An example relates, processing a continuous time signal x(t) to recovery of a signal component with energy in the vicinity of fc (Hertz) by first bandpass filtering the input signal x(t) retaining signal components in the vicinity of fc and then modulating the signal with a periodic signal m(t) with a period 1/fc. An analog continuous time (CT) filter using opamps is hard to control and not coherent to everything else. The CT filter is an IIR, which adds phase error. An example is shown in Tab A, FIG. 1.
When a bandpass filter is implemented in analog components, the phase and magnitude at the desired center frequency f may be difficult to control. For example, small changes in the actual center frequency of the filter may have relatively large effects on the phase at the desired (as opposed to the actual filter) center frequency. Therefore, even if the modulation frequency for the multiplexor is exactly fc, the unpredictable phase response of the filter may make it difficult to achieve coherent processing.
Modulator blocks are used in a variety of other applications at the input and/or output of analog filter blocks, for example, implemented as analog circuits or implemented using discrete time digitized sample signal processors.